Nancy Benson: Everybody yawns. We do it multiple times every day when we’re sleepy, just waking up or bored. We yawn when we see somebody else yawn. We yawn when we read about yawning. In fact, the odds are pretty good you may be yawning soon simply because you’re hearing us talk about it. Yet while yawning is one of those things everybody does… nobody knows why.

Dr. Robert R. Provine: Yawning feels good when we do it. People report that yawning on a one to ten scale rates about an 8.5, which is pretty impressive. So people enjoy yawning, whether it’s good for them or not.

Benson: Dr. Robert R. Provine (pro-vyne) is professor of psychology at the University of Maryland, Baltimore County. He’s studied yawning for more than 30 years, and is author of the book, Curious Behavior: Yawning, Laughing, Hiccupping and Beyond.

Provine: The actual yawn lasts the average of six seconds. They can be little shorter and sometimes much longer. But everyone yawns in pretty much the same way. One of the typical characteristics of yawning is the gaping mouth, the squinting of the eyes, the tilting back of the head, and there’s often stretching movements of other body parts. So a yawn is a very vigorous behavior that involves a lot of different parts of the body, like a total body stretch that focuses on the face.

Dr. Andrew Gallup: Yawning includes two primary components, that is the deep inhalation of air that accompanies the yawn and the strong stretching of the jaw and gaping of the jaw that occurs,

Benson: That’s Dr. Andrew Gallup, assistant professor of psychology at the State University of New York at Oneonta, who also conducted extensive research into yawning.

Gallup: In particular, this gaping of the jaw, stretching of the jaw promotes an increase in cerebral blood flow. So, basically stretching increases circulation and stretching of the skull is going to increase circulation to the skull.

Provine: Yawning being a major, powerful vigorous activity is causing a lot of things to happen in our body: blood pressure is going up, heart rate is going up, intracranial pressure is going up. It causes an opening of the Eustachian tubes; it’s helpful in clearing the ears. So, for example when you’re descending in an aircraft; for that matter, if you’re descending while scuba diving. So yawning is a powerful behavior that’s having a lot of different effects.

Benson: Still, experts aren’t sure why we yawn. And we can’t really look at the triggers for yawning, either… because there are so many of them.

Provine: We do yawn a lot when we are sleepy. Folklore is correct there. We yawn the most after waking in the morning and before bedtime in the evening. We yawn more when we are bored than when we are alert. But sometimes we’ll yawn a lot even when we are excited, such as athletes before a big event, a musician going out to play a concerto with an orchestra. These individuals certainly aren’t bored.

Benson: Provine says virtually all animals with a backbone yawn, from birds to snakes to humans. However, among people… yawning is incredibly contagious.

Provine: In fact yawning is so contagious that anything associated with it will cause a yawn. Those observing someone else yawn, or simply talking about it, talking about yawning, even thinking about or reading about yawning will trigger yawns.

Benson: A lot of people have heard that we yawn to make up an oxygen deficit. Makes sense, because we inhale deeply when we yawn. But Provine’s proven that that’s false.

Provine: You give people 100 percent oxygen to breath they don’t yawn less, if you give then enhanced levels of carbon dioxide they don’t yawn more. So that’s part of the folklore that we can dispose of. Some other functions that have been suggested are that yawning is a brain-cooling maneuver, and there’s some evidence for that. We seem to yawn more in the summer than in the winter or going from a cool area to a warmer area. So maybe yawning is a kind of air conditioning maneuver for the brain.

Benson: Gallup has done a lot of the work in this area. He says studies are piling up behind the theory that yawning could be part of the brain’s cooling system.

Gallup: In particular one is try to manipulate brain temperature by having participants apply either warm packs or cold packs to their forehead while watching contagious yawning videos. Interestingly enough when participants hold cold packs to their forehead yawning is completely diminished. However, when participants hold warm packs to their forehead indirectly altering the temperature of their brain, they yawn a significantly higher level.

Benson: In other studies, Gallup has implanted temperature probes into the brains of rats to constantly record temperature changes.

Gallup: What we found is that leading up to yawns there’s this striking rise in brain temperature that occurs about 60 seconds before the yawn is initiated, which amounts to about an increase of .1 degrees Celsius in brain temperature. And while that seems rather slight, it actually accounts for over twenty-five percent of the variation in brain temperature that occurs throughout the entire day. Immediately following that yawn, that peak of crest in temperature, following the yawn we see a corresponding decline in temperature back to baseline level.

Benson: So does that mean our brains get overheated when we’re tired, or when we’re waking up in the morning? Gallup says yes.

Gallup: Peak yawning occurs shortly after waking and before we go to sleep in the evening. In fact, most yawns occur in the evening before we go to sleep, but there’s also a rise in yawning shortly after we wake. It turns out that these correspond with changes in brain and body temperatures in these periods, in particular in the evening before we go to sleep brain and body temperatures are often at a relatively high point during the circadian rhythm and following sleep onset we see the decline in brain and body temperature. In fact, while we’re sleeping our brain and body temperatures are at their lowest point across the twenty-four hour cycle. Shortly after we wake we see quite a striking rise in brain and body temperature that coincides with this elevated yawning frequency as well.

Benson: The findings about why we yawn may serve as more than a curiosity. Gallup says they may also have medical uses. Some conditions such as multiple sclerosis and epilepsy often include an inability to regulate temperature, so they are prone to yawning.

Gallup: Frequent yawning could be used as a diagnostic indicator of thermal regulatory dysfunction or medical problem if it occurs in the absence of sleep problems or sleep deprivation. So that if you notice yourself yawning frequently that might actually be a sign of something that you might want to bring up to your physician and get it checked out or looked into.

Benson: Gallup says yawns are a physiological response that we can’t control, so if you think someone else is yawning out of boredom… think again. Any number of triggers can create an irresistible urge to yawn. In fact, Provine and Gallup say it’s likely you’ve yawned a few times listening to us talk about it.